Mirror selection not working properly
Try those two things:


Check if your pivot point is set properly. It has to be in the middle of your x-achis. To set your pivot hit SHIFT+CTRL+ALT+C (or hit SPACE and type "set origin"). Then choose "Origin to Geometry" or "Origin to Center of Mass".


Now your mirror selection should work correctly. If not, also do point 2:


Delete one half of your mesh. add a mirror modifier. mark your seems. Apply your mirror.